This invention relates in general to the identification of failed fuel assemblies in a nuclear reactor and in particular to the gas tagging method for fuel failure identification. More particularly, the invention relates to the development of designs to distinguish between single and multiple assembly failures.
The principles of gas tagging of fuel assemblies of a nuclear reactor for detecting fuel failures are well known in the art as may be seen by reference to U.S. Pat. No. 3,663,363, issued on May 16, 1972, to C. E. Crouthamel et al for "Identification of Failed Fuel Elements." In general, gas tagging consists of the addition to nuclear reactor fuel pins of small amounts of gas having a unique isotopic composition for each assembly; when an assembly fails during subsequent irradiation, the tag gas is released along with the fission gas and enables an identification of the defective assembly by a mass spectrometric analysis of the reactor cover gas. Isotopes of inert gaseous elements are normally chosen for the tag gas material because they are chemically inert, have minimal detrimental effect on the fuel pins in an assembly, and typically are non-fission products. A more complete description of methods of loading tag gases into reactor fuel pins is found in Ser. No. 403,309, filed Oct. 3, 1973, in the name of Ira V. Nelson.
Use of a ratio of two isotopic concentrations of a single element (i.e., a "one-ratio" or "one-dimensional" system) as provided in U.S. Pat. No. 3,663,363, supra, effectively eliminates the sensitivity of the mass spectrometer measurements to the chemical and gas dynamaic properties of the isotope. Earlier designs involving ratios of two isotopic concentrations of a single element have incorporated either one-, two-, or three- independent ratios. In order to obtain the maximum resolution of gas ratios for each single assembly, a uniform spacing in each tag ratio was customarily selected.
The difficulty with previous designs for tag gas selection is that there is no way to insure that failure of two or more assemblies will not be confused with that of a single assembly. The prior state-of-the-art, as represented by the work of Omberg and Schenter (Transactions of the American Nuclear Society 16, 215 (1973), is based upon intuitively shifting the gas tag ratio locations away from the normal positions in an orderly two- or three-ratio array in a Cartesian coordinate system. Such a design is limited for two reasons: It does not systematically insure that there will be no confusion between single and multiple failures, and the maximum number of failures occurring simultaneously which has been studied with such a scheme is only two.
Accordingly, it is the object of the present invention to provide a design for the arrangement of the ratios of isotopes of inert, tag gas that will insure that failure of a single assembly can be distinguished from that of more than one assembly.
A more specific object is to provide such a gas tag design which will not compromise the operability nor reliability of the identification of a single failed assembly.
Still another object is to provide such a gas tag ratio design that will function in a reactor with a cover gas comprised of a natural inert element.
Still another object is to provide a gas tag ratio design such that catastrophic failure of the gas tag system will not occur if the inert reactor cover gas of the reactor is contaminated with isotopes of the inert gas used for tagging.
A still further object is to provide a gas tag ratio design which will use existing, commerically available inert gases.
Other objects of the invention will in part be obvious and will in part appear hereafter.